I have a 99.5 Audi A4 1.8T Quattro and im trying to figure out if the hose that connect to the fuel rail is a return or feed line? Its the hose that is toward the front of the fuel rail nearest to the front bumper.

 

on my 98 the supply line is the forward one, on the fuel rail.
since the supply comes to the fuel rail pressurized, it would make sense.
the return line is the one that comes out of the bottom where the pressure regulator is. again makes sense as the regulator works by bleeding off pressure and returning it back to the tank

 

Correct. The one under the FPR is the fuel return line. As pressure is applied to the top of the FPR it closes allowing less fuel out of the rail and this causes the fuel pressure in the rail to rise.